My 4106 got a 7 row core from Macs radiator in Salem, OR about a month ago. They have bunches of locations in the PNW.

New core bolts to existing upper and lower "tanks" and has identical exterior dimensions. I overhauled my vernatherm and built a brand now shroud using aluminum and simple hand tools. I was planning to post some DIY threads on these topics but have been procrastinating

The tanks set the exterior dimensions. If you look, the finned part is narrower than the tanks. A 7 core is 1 inch less narrower (please pardon my horrible grammar) in the finned part than the 5 core. The thickness of the tanks (depth?) dictates how many cores will fit.

Also another option is a staggered tube setup for more cooling. Since I'm spending your money and not mine.

As has been mentioned, the 7 row core will fit using your existing top and bottom tanks. Number of fins/inch and the dimples improves the heat transfer.

Make sure the torus drive works properly. . . Several may disagree and recommend a straight drive, but if the torus is healthy?

Some folk have replaced the fan with a similar diameter but more blades. . .

Make sure the fan shroud does it's job - no places around it for air to escape. The fan should suck all the air thru the radiator, nothing from around it. There have been posts by GM and MCI owners about gaps causing problems. . .

Also, since you tend to stay in warmer climates, freezing is less of an issue. Therefore, you might consider using a 60/40 or 70/30 water/coolant mix ratio to improve hot weather cooling. As you probably know, straight water cools the best, but rust becomes an issue quickly! So you've got to maintain some ratio of coolant to reduce the crud factor.

Even though some don't like them, a good mister system has benefits when you could use some more cooling on a hot day on a long climb. They can be installed at minimal cost after you get your coach back on the road. There are many posts here to help fabricate one with locally available parts.